Launching Code on Document Ready

The first thing that most Javascript programmers end up doing is adding some code to their program, similar to this:

window.onload = function(){ alert("welcome"); }

Inside of which is the code that you want to run right when the page is loaded. Problematically, however, the Javascript code isn’t run until all images are finished downloading (this includes banner ads). The reason for using window.onload in the first place is that the HTML ‘document’ isn’t finished loading yet, when you first try to run your code.

To circumvent both problems, jQuery has a simple statement that checks the document and waits until it’s ready to be manipulated, known as the ready event:

Save your HTML file and reload the test page in your browser. Clicking the link on the page should make a browser’s alert pop-up, before leaving to go to the main jQuery page.

For click and most other events, you can prevent the default behaviour – here, following the link to jquery.com – by calling event.preventDefault() in the event handler:

$(document).ready(function(){
$("a").click(function(event){
alert("As you can see, the link no longer took you to jquery.com");
event.preventDefault();
});
});

Complete Example

The following is an example of what the complete HTML file might look like if you were to use the script in your own file. Note that it links to Google’s CDN to load the jQuery core file. Also, while the custom script is included in the <head>, it is generally preferable to place it in a separate file and refer that file with the script element’s src attribute

Adding and Removing an HTML Class

Important:The remaining jQuery examples will need to be placed inside the ready event so that they are executed when the document is ready to be worked on. See Launching Code on Document Ready above for details.

Another common task is adding (or removing) a class.

First, add some style information into the <head> of your document, like this:

CALLBACK AND FUNCTIONS

A callback is a function that is passed as an argument to another function and is executed after its parent function has completed. The special thing about a callback is that functions that appear after the “parent” can execute before the callback executes. Another important thing to know is how to properly pass the callback. This is where I have often forgotten the proper syntax.

Callback without arguments

For a callback with no arguments you pass it like this:

$.get('myhtmlpage.html', myCallBack);

Note that the second parameter here is simply the function name (but not as a string and without parentheses). Functions in Javascript are ‘First class citizens’ and so can be passed around like variable references and executed at a later time.

Callback with arguments

“What do you do if you have arguments that you want to pass?”, you might ask yourself.

Right

The problem with the above example is that myCallBack(param1, param2) is evaluated before being passed as a function. Javascript and by extension jQuery expects a function pointer in cases like these. I.E. setTimeout function.

In the below usage, an anonymous function is created (just a block of statements) and is registered as the callback function. Note the use of ‘function(){‘. The anonymous function does exactly one thing: calls myCallBack, with the values of param1 and param2 in the outer scope.

$.get('myhtmlpage.html', function(){
myCallBack(param1, param2);
});

param1 and param2 are evaluated as a callback when the ‘$.get’ is done getting the page.

A basic introduction to jQuery and the concepts that you need to know to use it.